A vacuum pump having the above-described pumping performance is used in a sputtering apparatus, a helium leak detector, an analyzer (e.g., SEM), and the like. Further, the vacuum pump having the above-described pumping performance is used as a roughing vacuum pump coupled to a high-vacuum pump, such as a turbo molecular pump, and is also used in a vacuum dryer or a vacuum bonding apparatus that sucks a gas, e.g., water vapor.
An oil-sealed rotary pump is typically used as a small-volume vacuum pump which is small size, portable, and inexpensive. However, this type of pump presents several problems including back diffusion of oil vapor into a vacuum chamber (contamination of the vacuum chamber and a workpiece), oil contamination of discharge lines (fire hazard), environmental pollution, a decrease in oil due to the oil diffusion, deterioration and fouling of oil as a result of oil contamination with moisture, and the need for routine supply and replacement of oil. Particularly, fluorine oil for use in the vacuum pump is expensive, and the oil replacement necessitates troublesome operations.
Thus, a scroll-type dry vacuum pump has recently been used as a substitute for the oil-sealed rotary pump. The scroll-type dry vacuum pump is oil-free, and has a scroll that provides a scrolling motion for continuously compressing a gas from vacuum to atmospheric pressure. This type of pump has advantageous features, such as a small driving power during an evacuating operation and a relatively high performance in view of ultimate pressure (about 1 Pa). However, this pump uses tip seals (contact seals) on edges of the scroll. These tip seals wear, producing particles which would contaminate the inside space of the vacuum chamber. Further, wear of the tip seals causes a fall in a pumping performance with time, and thus replacement of the tip seals is required as a result of continuous pump-operation for about one year. Moreover, replacement of the tip seals entails dismantling of the vacuum pump, which results in an increased replacement cost. The scroll-type dry vacuum pump also has problems of being large in size and high cost, compared with the oil-sealed rotary pump having the same volume.
In addition, because the scroll-type dry vacuum pump has the scrolling mechanism which provides compression processes, when evacuating a gas under the atmospheric pressure, compression of the gas becomes excessive, and consequently a large driving power is required. Further, the scroll-type dry vacuum pump has several drawbacks, such as condensation of water vapor due to the compression process, a difficulty in compensating an unbalance of a rotary member, and excessive vibration.
Thus, it has been customary in the semiconductor fabrication process to use a multistage Roots-type or screw type positive displacement dry vacuum pump having a relatively high pumping speed (at least 1000 L/min). In order to achieve ultimate pressure of not more than 1 Pa and a higher pumping speed, a main pump and a booster pump may be coupled in series via a pipe to constitute a single vacuum pump unit. The two-axis positive displacement dry vacuum pump does not use oil in gas passages thereof, and thus does not cause the oil contamination. Further, the positive displacement dry vacuum pump does not use the tip seals, and hence non-contact operation can be realized. These are the most remarkable features of the positive displacement dry vacuum pump.
However, this type of pump uses timing gears for synchronizing rotations of a pair of pump rotors in opposite directions with a small clearance maintained between the pump rotors, i.e., for allowing non-contact synchronized rotation in the opposite directions. Because the timing gears require oil lubrication, the above vacuum pump is not an oil-free pump, and necessitates maintenance, such as oil replacement. In addition, in order to prevent back contamination of oil vapor into gas passages, it is necessary to use N2 gas or the like to purge the oil vapor.
When the gas, to be evacuated, is of the atmospheric pressure, the booster pump is not operated or the booster pump is operated at a low rotational speed, so that an excessive load is not applied thereto. In the main pump, a cooling device is generally provided for cooling heat of compression. Consequently, the operation of the two-axis positive displacement dry vacuum pump generally requires supply systems of cooling water, N2 gas, and the like, in addition to electric power.
When evacuating a relatively high temperature gas containing water vapor for the purpose of vacuum drying or the like, the following problems would arise. Pressure at the inlet side of the vacuum pump unit is low, and therefore, the water vapor in the gas is maintained as it is. On the other hand, the outlet side is under the atmospheric pressure, and therefore the water vapor condenses on relatively low temperature portions of gas passages in the vacuum pump unit. As a result, the condensed water flows into the pump. Such water not only applies an excessive load to the pump, but also contaminates lubrication oil of bearings, causing a lowered lubrication performance that would greatly shorten the life of the pump.
Furthermore, arrangements of the pumps, pipes, and valves, each constituting the vacuum pump unit, are complex. Such complex arrangements would lower the efficiency of piping operations, and would also cause misconnecting of the pipes and operation failure of the vacuum pump unit.